Metal materials coated with thermoplastic polymer film are well known. The use of such materials for rigid metal packaging has been widely described.
Simultaneous lamination of polyester and polyolefin films to metal sheet is disclosed in EP-A-0312302. This specification is primarily concerned with lamination to steel sheet of a composite polyester film on one major surface and a polyolefin containing film on the other major surface. The polymer film coated steel sheet was found to be useful in the production of can ends.
Simultaneous lamination of a polyester/metal/polyester laminate is also disclosed in EP-A-0312303 in which a composite coextruded polyester film is adhered to steel sheet. Good corrosion resistance was imparted by the use of an outer polyester layer having a crystallinity greater than 30% as measured by density.
EP-A-0312304 describes a polymer film coated metal sheet to which is adhered an amorphous polyester. The polymer film coated metal sheet was found to be particularly useful in the formation of drawn and wall ironed cans made from aluminium 3004 alloy.
JP 58-25591 describes a process for the production of a metal substrate coated with a thermoplastic polyester resin using low crystallinity starting film material. It was found that a crystallinity range of 5 to 50% for the polyester in the laminate was useful because the forming and processing properties of coated metals and the corrosion resistance of the formed, processed products were dramatically increased by maintaining the degree of crystallisation of the resin layer to within the specified limits.
GB-A-2123746 discloses a method of producing a metal sheet laminated with a polyester resin film by coating the metal with a crystalline and oriented polyester resin film. Laminates of steel or aluminium were produced by a controlled single step, metal heating process, film application and then without reheating, quenching.
Techniques for the manufacture of beverage can ends are well established.
EP-A-0153115 discloses a method of making a can end. According to this method, a disc is blanked and a shallow drawn cup is formed from the disc. The shallow drawn cup is reformed into a shell by forming a centre panel, allowing the wall of the cup to fold and producing a countersink or reinforcing channel. The presence of the countersink is acknowledged to be important in rendering the can end resistant to internal carbonation pressure. In a typical can end, the profile of the countersink and the size of the countersink radius limit the geometric strength. In general, the smaller the countersink radius, the stronger the can end in resistance to outward movement when pressurised as part of a filled can.
Conventionally, a material which has hitherto been extensively used commercially for easy open beverage can ends is lacquer coated aluminium alloy AA5182. This alloy has a high magnesium content of around 4 to 5% and the high magnesium content of the 5182 alloy gives it high strength. Because of its strength and good formability, this alloy confers excellent pressure resistance, typically 100 psi to can ends for a 5182 gauge of 0.28 mm. A problem with the 5182 alloy is that, because of its high strength, the cost to produce material for can ends is relatively large. In particular, the high magnesium content adds to the material cost and increases the mill power, mill energy consumption and time required to roll 5182 to can endstock gauges compared to a lower magnesium content alloy such as AA3004. Both factors, combined with a poorer metal yield for 5182, result in a higher manufacturing cost for 5182 than 3004.
In order to reduce the cost of producing the can ends, lacquered 5182 alloy can be made to thinner gauge. However, thinner gauge 5182 alloy is less resistant to pressure. The pressure resistance of can end shells produced from thinner gauge 5182 can be increased to acceptable values (90 psi) by deepening the countersink depth. For 0.245 mm 5182 the countersink depth must be increased from the standard 0.250 inches to 0.270 inches, however the end is no longer seamable to cans without a change in the seamer operation. Ends not interchangeable on seaming with standard ends used in the beverage industry are not desirable.
Attempts to increase the pressure resistance of thinner gauge 5182 by tightening the countersink radius of the can ends are comparatively ineffective (see FIG. 8) and may result in a new problem. Cracks appear in the reformed lacquered 5182 material which in production could result in an unacceptable level of end metal failures and, if used commercially, would result in corrosion of the can end with consequential damage to the contents of the can.
In order to overcome the problems of cost and interchangeability, it has been surprisingly found that thermoplastic polymer film coated aluminium alloys of lower magnesium content than 5182 and lower strength than 5182 can be used to form can ends having a countersink radius of sufficient tightness to confer upon the can ends commercially useful pressure resistances.